Electronic paper is proposed to be applied to various portable equipment, such as electronic books, sub-display of mobile terminal equipment and display portion of IC cards. One promising display device for electronic paper is a display device using liquid crystal composition in which a cholesteric phase is formed (referred to as cholesteric liquid crystals or chiral nematic liquid crystals, which are generically referred to as cholesteric liquid crystals in the present description). Cholesteric liquid crystals have excellent characteristics, including a semi-permanent display holding characteristic (memory characteristic), clear color display characteristic, high contrast characteristic and high resolution characteristic.
FIG. 1 is a diagram depicting a cross-sectional configuration of a liquid crystal display device using cholesteric liquid crystals that can display full color. The liquid crystal display device 1 has a blue display portion 10, green display portion 11 and red display portion 12, which are layered sequentially from the display surface at the user 3 side. The upper substrate side in FIG. 1 is a display surface, and external light 2 enters from the area above the substrate to the display surface.
The blue display portion 10 has blue liquid crystals 10LC sealed between a pair of top and bottom substrates 10A and 10B, and a pulse voltage supply 10P which applies a predetermined pulse voltage to the blue liquid crystal layer 10LC. The green display portion 11 has green liquid crystals 11LC sealed between a pair of top and bottom substrates 11A and 11B, and a pulse voltage supply 11P which applies a predetermined pulse voltage to the green liquid crystal layer 11LC. The red display portion 12 has red liquid crystals 12LC sealed between a pair of top and bottom substrates 12A and 12B, and a pulse voltage supply 12P which applies a predetermined pulse voltage to the red liquid crystal layer 12LC. A light absorption layer 13 is disposed on the back surface of the bottom substrate 12B of the red display portion 12.
The cholesteric liquid crystals used for each blue, green and red liquid crystal layer 10LC, 11LC and 12LC, are a crystal mixture in which a relatively large amount of chiral additives (also called chiral material) is added to nematic liquid crystals at several tens wt % of percentage content. If a relatively large amount of chiral material is contained in the nematic liquid crystals, the cholesteric phase, in which nematic liquid crystal molecules are strongly twisted in spirals, can be formed. Consequently the cholesteric liquid crystals are also called chiral nematic liquid crystals.
Cholesteric liquid crystals have a bi-stable characteristic (memory characteristic) and can be in a planar state (reflection state), focal conic state (transmission state) or an intermediate mixed state thereof by adjusting electric field strength to be applied to the liquid crystals. Once the cholesteric liquid crystals enter the planar state, focal conic state or intermediate mixed state thereof, the state is stably held even if the electric field disappears thereafter.
The planar state is generated, for example, by applying a strong electric field to the liquid crystal layer by applying a predetermined high voltage between the top and bottom electrodes, so that the liquid crystals becomes a homeotropic state, then rapidly decreasing the electric field to zero. The focal conic state is generated, for example, by applying a predetermined voltage which is lower than the above mentioned high voltage between the top and bottom substrates, so as to apply an electric field to the liquid crystal layer, then rapidly decreasing the electric field to zero. The focal conic state can also be generated by gradually applying voltage from the planar state. The intermediate state between the planar state and the focal conic state is generated, for example, by applying a voltage lower than the voltage for generating the focal conic state, between the top and bottom substrates, so as to apply an electric field to the liquid crystal layer, then rapidly decreasing the electric field to zero.
FIG. 2A and FIG. 2B are diagrams depicting the display principle of a liquid crystal display device using cholesteric liquid crystals. In FIG. 2A and FIG. 2B, the blue display portion is described as an example. FIG. 2A depicts the orientation state of liquid crystal molecules LC of the cholesteric liquid crystals when the blue liquid crystal layer 10LC of the blue display portion 10 is in the planar state. As FIG. 2A depicts, the liquid crystal molecules LC in the planar state sequentially rotate in the substrate thickness direction and form a spiral structure, and the spiral axis of the spiral structure is roughly vertical to the substrate surface.
In the planar state, lights having a predetermined wavelength, according to the spiral pitch of the liquid crystal molecules, are selectively reflected by the liquid crystal layer. If an average refractive index of the liquid crystal layer is n and the spiral pitch is p, the wavelength λ with which the reflection is the maximum is given by λ=n·p. Therefore if the average refractive index n and the spiral pitch p are determined so that λ=480 nm is established, for example, then the blue liquid crystal layer 10LC of the blue display portion 10 selectively reflects the blue lights in the planar state. The average refractive index n can be adjusted by selecting the liquid crystal material and chiral material, and the spiral pitch p can be adjusted by adjusting the percentage content of the chiral material.
FIG. 2B depicts the orientation state of the liquid crystal molecules of the cholesteric liquid crystals when the blue liquid crystal layer LC of the blue display portion 10 is in the focal conic state. As FIG. 2B depicts, the liquid crystal molecules in the focal conic state sequentially rotate in the substrate plane direction and forms a spiral structure, and the spiral axis of the spiral structure is roughly parallel with the substrate surface. In the focal conic state, the blue liquid crystal layer 10LC loses the ability to select the reflection wavelength, and most of the incident lights 2 transmit. Since the transmitted lights are absorbed by the light absorption layer 13 disposed on the back surface of the bottom substrate 12B of the red display unit 12, the screen becomes a dark (black) display.
In the intermediate state between the planar state and focal conic state, the ratio of the reflected light and transmitted light can be adjusted according to the state, so the intensity of the reflected light can be changed. Thus in the case of the cholesteric liquid crystals, reflected light quantity can be controlled by the orientation state of the liquid crystal molecules twisted into a spiral.
If cholesteric liquid crystals, which selectively reflect green or red light in the planar state, are sealed in the green liquid crystal layer and red liquid crystal layer respectively, just like the above mentioned case of the blue liquid crystal layer, a full color liquid crystal display device can be implemented.
By layering liquid crystal display panels, which selectively reflect red, green and blue light using cholesteric liquid crystals like this, a full color display device which has the memory characteristic can be implemented, and a color display with zero power consumption becomes possible, except for when the screen is refreshed.
In the case of a display device utilizing the selective reflection of cholesteric liquid crystals, however, it is necessary to temporarily reset the liquid crystals to the homeotropic state with high voltage when the display image is changed. This means that high power is required during reset. This high power required during reset is a major problem for portable equipment, of which the instantaneous capacity of a battery is limited.
A display refreshing method to solve this problem is disclosed in the following WO06/103738 (2006.10.05 International Publication). According to this display refreshing method of Patent Document, reset driving is executed by simultaneously selecting a plurality of scan electrodes, and a group of a plurality of scan electrodes being selected is scanned. Since reset driving is limited to part of the scan electrode group on screen, instantaneous power can be suppressed. Also reset driving is performed to the plurality of scan electrodes simultaneously, so the time to reset one screen can be decreased.
FIG. 3 depicts a problem of a conventional display refreshing method. In the display state depicted in FIG. 3, the English language display screen “A B C D . . . ” 20 is gradually changed to the Japanese language display screen “a i u e o . . . in Japanese” 21, 22, 23. According to the display refreshing method of WO06/103738 (2006.10.05 International Publication), a plurality of scan electrodes are simultaneously selected and driven for reset, and the selected scan electrode group is scanned. Therefore the user can recognize the state where a strip area 24 being reset (or being reset and written) moves in the scan direction.
This strip area 24 is in a homeotropic state, and this homeotropic state, which is transparent, is recognized as a strip in a color of the light absorption layer 13, normally black, by the user. Time when the strip area 24 is recognized depends on the number of lines of the scan electrode, but is time required for resetting all the pixels and is approximately several seconds to several tens of seconds. Since the display of this strip area 24 is irritating to a user, decreasing this irritation is demanded.
Also it takes about several seconds to several tens of seconds to refresh the display image that includes reset, so the image is always refreshed in the same scan direction whether the image data direction is written vertically or horizontally, which also is irritating to the user.